<p>Vanadium oxyphosphate (VOPO₄) is a promising cathode material for zinc-ion batteries (ZIBs), but its capacity is critically limited by the irreversible oxidation of electrochemically active V⁴⁺ to inactive V⁵⁺ during cycling. To address this challenge, we introduce 2-hydroxyphosphonoacetic acid (HPAA) as a multifunctional additive for polyacrylamide (PAM) gel electrolyte. Leveraging its reducing properties, HPAA effectively preserves a higher proportion of V⁴⁺ with redox activity during charge/discharge process. X-ray photoelectron spectroscopy (XPS) analysis confirms this interfacial modulation by HPAA, revealing increased concentration of V⁴⁺ and decreased concentration of V⁵⁺ on cycled cathodes, indicating a more reactive surface. Furthermore, HPAA lowers the reaction energy barrier for the V⁴⁺/V⁵⁺ redox couple and mitigates vanadium dissolution, collectively optimizing the reaction kinetics. Electrochemical impedance spectroscopy (EIS) analysis with the equivalent circuit confirms that HPAA reduces ion diffusion resistance, boosting the Zn²⁺ diffusion coefficient to 7.0676 cm² s^− 1. Consequently, the HPAA-modified PAM gel electrolyte enables VOPO₄ cathode delivers a significantly enhanced discharge capacity of 387 mAh g^− 1 at 0.1&#xa0;A g^− 1 and retains 163 mAh g^− 1 after 100 cycles. This work unveils a novel strategy utilizing organophosphonic reductants to regulate vanadium valence states in cathode interface, providing crucial insights for designing high-capacity ZIBs.</p>